The present invention relates to nonvolatile memories using memory devices such as a device called a one time programmable (OTP) device capable of rewiring stored information only once using a fuse.
With recent progress of miniaturization in fabrication processes of semiconductor technology, the thickness of gate oxide films has been reduced and gate electrode materials have been modified. Accordingly, the scale and integration degree of devices (memories) for rewriting, such as FLASH memories and electrically erasable programmable read-only memories (EEPROMs), have been increased, resulting in achievement of technical progress. These types of nonvolatile memories generally have high demands for the reliability of data held for a long period and, in addition, special processes other than standard CMOS (complementary metal oxide semiconductor) processes are needed for, for example, formation of floating gates. Accordingly, the fabrication cost tends to increase.
On the other hand, the area of systems using semiconductor devices includes a field in which the purposes of necessary devices have been changing. In such a field, nonvolatile memory devices or OTP devices are more and more often incorporated in products, as seen in products for security use and IC tags, for example. In existing systems, memories incorporated in system LSI and analog devices, for example, redundancy replacement using fuses in a wafer process or readjustment after packaging is needed in some cases such as redundancy addition and calibration of an analog circuit. In particular, if a device exhibits high performance with respect to frequency, adjustment in a subsequent process is inevitably needed in some cases because of a mismatch between a low-frequency test in a probe process on a wafer and an actual-frequency test after packaging. That is, to compensate for decrease in adjustment accuracy due to the influence of placing a probe itself, fine adjustment after packaging is effective in some cases.
As recent OTP fuses for applications as described above, existing laser fuses which are made of metal or polysilicon and blown with laser repair apparatus or electric fuses which are blown by causing current to flow in fuse portions are being used in view of advantages in which fabrication by standard CMOS processes is possible and reliability of written data is high, while the number of writing is limited to one (see, for example, U.S. Pat. No. 6,384,664).
As a technique that enables writing to be performed multiple times while keeping the advantages of OTP devices as described above, a technique of causing a once-written state to be inverted by performing exclusive OR (XOR) operation on a plurality of OTP devices is known (see, for example, U.S. Pat. No. 5,966,339)
The configuration in which XOR operation is performed so as to enable repeated writing, however, needs XOR circuits provided in number according to the number of possible rewriting or the number of bits, so that this configuration has a problem in which the circuit scale increases.